Protein B23 is an abundant, multifunctional nucleolar
phosphoprotein whose activities are proposed to play a
role in ribosome assembly. Szebeni et al. (1997) showed
stimulation of nuclear import in vitro by protein B23 and
suggested that this effect was due to a molecular chaperone-like
activity. Protein B23 was tested for chaperone activities
using several protein substrates. The temperature-dependent
and -independent aggregation of the HIV-1 Rev protein was
measured using a zero angle light scattering (turbidity)
assay. Protein B23 inhibited the aggregation of the Rev
protein, with the amount of inhibition proportional to
the concentration of B23 added. This activity was saturable
with nearly complete inhibition when the molar ratio of
B23:Rev was slightly above one. Protein B23 also protected
liver alcohol dehydrogenase (LADH), carboxypeptidase A,
citrate synthase, and rhodanese from aggregation during
thermal denaturation and preserved the enzyme activity
of LADH under these conditions. In addition, protein B23
was able to promote the restoration of activity of LADH
previously denatured with guanidine-HCl. Protein B23 preferentially
bound denatured substrates and exposed hydrophobic regions
when complexed with denatured proteins. Thus, by several
criteria, protein B23 behaves like a molecular chaperone;
these activities may be related to its role in ribosome
biogenesis.